The p53 tumor suppressor gene product is a key mediator of cell cycle arrest and apoptosis in response to genotoxic stress. These responses are essential to prevent the emergence of transformation-prone cells with unstable genomes. The biological role of p53 as a DNA damage checkpoint is directly attributable to its biochemical function as a sequence-specific transcriptional activator. Indeed, nearly all cancer-associated p53 mutations abrogate this function. The transactivation function of p53 requires accessory protein complexes, the coactivators, such as the p300/CBP family. We have identified hADA complex, the homologue of the yeast ADA complex, as a novel coactivator of p53. We hypothesize that histone acetyl transferase (HAT)-containing hADA complex, either by itself or in concert with p300/CBP, regulates either the extent or the nature of p53-mediated DNA damage response by acetylating p53 and chromatin-associated histones in p53 target promoters.To address these hypotheses, we will first characterize the components of human ADA complex in cells under study, determine if ADA3 is the component that recruits ADA complex to p53, as our preliminary results suggest, and use mutagenesis to map the ADA3 and p53 domains, that mediate their interaction. We will use overexpression and dominant-negative approaches to investigate if ADA complex functions as a coactivator for p53, and determine if coactivator function relates to histone acetylation on p53 target gene promoters and/or p53 acetylation. Finally, we will quantify p53-mediated cell cycle arrest and apoptotic responses to DNA damage to assess the role of ADA complex versus p300/CBP in regulating p53-mediated DNA damage response, using overexpression as well as dominant-negative strategies. A successful outcome of the proposed studies should identify a novel mechanism for regulating p53-mediated DNA damage response, and provide potential targets for future development of diagnostic agents and rational therapeutics for human cancer.